This invention pertains generally to a method for monitoring the power distribution of a nuclear reactor and more particularly to a nuclear reactor monitoring method employing movable in-core neutron detectors.
In many of the reactors presently in operation, reliance for neutron flux level measurements, as used in the reactor control and protection systems, has been placed on out-of-core detectors supplemented by the in-core flux mapping system which has been in use since as early as 1967; with the latter intended to provide proofs of core design and some calibration measurements.
With a growing trend toward larger reactor cores, there has been some concern as to the possible adverse effects of core power distributions and whether out-of-core detectors could adequately detect such possible adverse distributions. Test data showed that the split section out-of-core detectors, presently in use, responded to axial flux tilts, but the accuracies of the tilt measurements were affected by the geometry and construction materials employed at the detector wells and by the spacing between the vessel and the detectors. Correction factors were developed for these effects, but there is still some question as to whether out-of-core detectors will in all cases provide adequate alarm of adverse power distributions.
A concerned governmental agency has suggested that a monitoring and alarm system utilizing fixed in-core detectors be provided for plants with a design F.sub.Q (the ratio of the peak local power to the average core power density) below the value of 2.50, so that the operator can be afforded continuous surveillance of the axial distribution. The governmental position includes a statement that the required system should be a fixed in-core detector system or "a better system".
A relative evaluation of performance of the fixed detector system is obtained by comparison with alternative systems which are currently performing, or could be developed to perform, the same function. The alternative systems considered include: the out-of-core long ion chamber system; the ion chamber system augmented by a third section; and selected movable in-core detector signals frequently taken.
The desirability of any surveillance system will depend first upon its accuracy ; secondly, upon its availability when reactor limits are approached; and thirdly, upon its usefullness to the plant operator as a tool for indicating the best mode of corrective action to be taken.
A fixed in-core system will meet only the second criterion. It has been shown, however, that frequent use of a movable in-core detector system can provide the most useful approach to monitoring the core average axial peaking factor, F.sub.Z.